Metal detection by eddy current probes has long been known in connection with the search for mines. A search coil system of an eddy current probe, which has a transmitting coil connected to an alternating current source and at least one receiving coil, is moved over the ground in the vicinity thereof. On approaching a piece of metal in or on the ground, eddy currents are induced in the metal by the electromagnetic alternating field of the transmitting coil, which act by mutual induction on the receiving coil and there produce a coil signal corresponding to an a.c. voltage. This is supplied to a signal evaluating device and evaluated. A following display or indicating means can optically and/or acoustically indicate or display the presence of a metal piece in the vicinity of the search coil system. The search area can then be more precisely investigated for the presence of a mine.
In practice, considerable difficulties arise when seeking mines, e.g. due to the fact that the search objects are hidden in magnetizable and/or electrically conductive soils, whose conductivity may be further increased by salt water. The soil or ground material can produce disturbance variables as a result of its characteristics which can be up to 500 times greater than the wanted signal caused by a metal piece.
Special operating and evaluating methods have been developed with which it is possible to suppress such unwanted signals. Examples of methods for suppressing unwanted signals in the case of metal detection in disturbing environments are described in international patent application WO 87/04801 and Swedish specification 82 02 094-2 and U.S. Pat. No. 4,563,644 which claims priority from the referenced Swedish specification. The basic "two-frequency method", in which the transmitting coil is excited with two discrete frequencies, is described in conjunction with FIG. 1.
Whilst the two-frequency method yields good results if the environment is exclusively magnetizable or substantially only electrically conductive, but not magnetizable, the case frequently occurs in practice where a soil is both magnetizable and also electrically conductive, in which case limits are encountered in the capacity to suppress ground or soil signals.
In addition, hitherto it has not been possible to gather from the coil signal information as to the nature of the metal involved, the size of the metal piece and the depth which it is buried in the ground.
Another major difficulty in connection with seeking mines by eddy current probes is the false alarm rate. In a search area where mines are assumed to exist, there are also frequency harmless metal objects distributed over or in the ground such as fragments, projectiles, cartridge cases, nails, crown corks, etc. Each of these metal articles can give rise to a search signal in the eddy current probe, without it being possible to decide whether the signal is attributable to a harmless object or a mine. This is particularly problematical in seeking plastic mines which, apart from a plastic body filled with explosive explosive only have a few, small metal parts. These can on the one hand only be detected with sensitive eddy current probes, but on the other, due to their small volume, produce similar signals to those of said small harmless metal objects. In order to obviate a hazard by plastic mines, it has hitherto been necessary when a metal signal occurs to investigate the search area more accurately and optionally manually for the presence of a plastic mine. This greatly slows down the search for mines and therefore reduces the effectiveness of the search.
An object of the invention is to provide a method for the operation of an eddy current probe and for the evaluation of the coil signals, which obviates the aforementioned disadvantages. It is in particular to be possible to carry out an identification of metal pieces, e.g. according to the nature of the metal. Preferably also the effectiveness of the mine search in the presence of plastic mines is to be increased: